This invention relates in general to integrated circuit packages and in particular, to a double encapsulation structure and method wherein a protective material is applied over a die, a portion of a lead frame and connecting bonding wires to form a first encapsulation structure which is in turn, encapsulated in an integrated circuit molded package.
One means of packaging an integrated circuit ("IC") is to encapsulate the IC in a plastic material. For example, in a plastic leaded chip carrier ("PLCC") package, the IC is electrically connected to an alloy 42 or copper lead frame by bonding wires and subsequently, the IC, bonding wires and portions of the lead frame are encapsulated by transfer molding techniques using a mold and a thermal set epoxy plastic. The resulting molded, plastic package is popular for commercial applications, because it is rugged, inexpensive and suitable for large scale production.
Reliability problems, however, can result from the fabrication process for molded, plastic IC packages. First, the bonding wires can be bent by an action referred to as "wire sweep." Wire sweep occurs when the bonding wires are laterally bent by viscous forces caused by the motion of the plastic material when it is being injected by a plunger into the IC package mold.
Wire sweep can result in two reliability problems. One problem is that adjacent wires may be bent towards each other in such a way as to cause the two wires to short. The second is that wire sweep causes longitudinal stress on the wire bonding points on both the IC and the lead frame, thus causing the bonding wires to pull away and potentially break electrical contact.
Differences in the thermal coefficients of expansion of the plastic molding compound and other parts of the packaged device can also result in reliability problems. For example, exposure to heat can cause the plastic compound to expand resulting in longitudinal forces pulling the bonding wires away from their bonding points on both the IC and the lead frame, thus potentially causing electrical contact breaks.
FIG. 1A illustrates a cross-sectional view of a typical PLCC wherein longitudinal forces 22 around radial axis 12 are caused by the plastic material 90 expanding. The longitudinal forces 22 pull on bonding wires 20 which are connected at one end to an IC 10 sitting on pad 30 and at the opposite end to a lead frame 40 at bonding points 24 and 26, respectively. Clearly, if the longitudinal forces 22 are sufficiently strong enough, the bonding wires 20 could break physical and electrical contact with either or both the IC 10 and lead frame 40 at the bonding points 24 and 26.
Conversely, exposure to cold can cause the plastic material to contract resulting in longitudinal forces pushing down on both the bonding wires and the IC surface, thus potentially causing fractures at both the bonding points and the IC surface. FIG. 1B illustrates a cross-sectional view of a typical PLCC wherein longitudinal forces 28 around radial axis 12 are caused by the plastic material 90 Contracting. The longitudinal forces 28 push down on bonding wires 20 which are connected at one end to the IC 10 sitting on pad 30 and at the opposite end to the lead frame 40 at bonding points 24 and 26, respectively.
If the longitudinal forces 28 are sufficiently strong, the bonding wires 20 could bend causing shearing forces 27 at bonding points 24 and 26. These shearing forces 27, in turn, could cause the bonding wires 20 to break physical and electrical contact with either or both the IC 10 and lead frame 40 at the bonding points 24 and 26. Additionally, if the longitudinal forces 28 are sufficiently strong enough, fractures could occur on the die surface 25 or at bonding points 24 and 26.
Prior attempts to alleviate these reliability problems associated with molded, plastic IC packages have not been entirely successful, and suggest even less promise for solving the reliability problems associated with the higher density ICs of the future. For example, to alleviate the problem of wire sweep, attempts have been made to use less viscous plastic molding compounds. Although these compounds reduce the viscous forces acting against the bonding wires when the compound is injected into the plastic IC package mold, they prove inadequate to reliably solve the increasing sensitivity to wire sweep caused by advances in IC technology such as decreases in the dimensions of ICs and increases in the required number of bonding wires.
Attempts to alleviate problems associated with differences in thermal coefficients of expansion between the plastic compound and other parts of the package device have also been less than entirely satisfactory. For example, the IC die surface has been coated with various materials to act as shock absorbers against the longitudinal forces pushing down on the die surface as a result of the plastic compound contracting from exposure to cold. The die surface coating, however, does not protect the bonding wires from longitudinal forces caused by the plastic compound either expanding or contracting. Thus, reliability problems still persist whereby the bonding wires may be pulled away from their bonding points on both the IC and the lead frame, thus causing electrical contact breaks, or the bonding wires may be pushed down onto their bonding points, thus causing fractures at these points.